Automatic detection system for broken tools in CNC machining centers using advanced machine vision techniques

Abstract

The invention describes a method for the inspection of machining tools used in computational numerical control (CNC) machining centers. The invention will observe a machining tool immediately after use to determine if it has been damaged. The invention is based on a machine vision technology that uses one or more image sensors to acquire, locate and compare the tool to a good model. The inspection equipment is designed for implementation in the CNC automatic tool changing system and will alert the operator that tool has been damaged before the next operation. This immediate response of the invention will contain defect components and prevent damage to subsequent components. With the addition of an infrared image sensor the invention is capable of detecting worn or dull tools that may damage a component due to excessive heating.

1. An apparatus with protective environmental enclosure for measurement and determination of key characteristics of machine cutting tools used in a CNC machining center with a machine vision measuring system comprising:
 a) an image processing computer for image acquisition, image storage and image processing capability; 
 b) the image processing computer comprising memory for storing images formed by a camera; 
 c) the image processing computer also comprising digital parallel input/output, digital serial, and ethernet communication capabilities for providing messages to external devices to report one or more measurements or characteristics of the machine cutting tool; 
 d) the image processing computer executing control software stored in a computer readable medium, for allowing request and response signals from external devices indicating a machine cutting tool to be inspected, for causing the image processing computer to perform image alignment and analysis for extraction of key characteristics of the machine cutting tool, as well as for causing the image processing computer to store a reference image of a new and unused machine cutting tool in a memory location referenced by a specific identification code that is unique to a specific machine cutting tool model or type, as well as causing a determination of the exact position of the machine cutting tool by extracting one or more edges or edge points based on grayscale sub-pixel information; 
 e) a camera having one or more image sensors and a lens for providing a spatial resolution and depth of field necessary to form a sharp focus image of a machine cutting tool or a target portion thereof including the cutting tool tip; 
 f) wherein each image sensor comprises sufficient pixel resolution to resolve an imperfection in a profile of the machine cutting tool or the target portion thereof including the cutting tool tip; 
 g) an LED backlight illumination system positioned on the opposite side of the machine cutting tool from the camera that forms a silhouette of the machine cutting tool; 
 h) a front lighting illumination system positioned on the same side of the machine cutting tool as the camera that highlights surface characteristics of the machine cutting tool; 
 i) a motorized translation stage for moving the camera and the LED backlight and front lighting illumination systems parallel to an axis of rotation of the machine cutting tool and maintaining an orientation of an optical axis perpendicular to the axis of rotation of the machine cutting tool; 
 j) a protective enclosure containing the camera image sensors, the lens, both the LED backlight and the front lighting illumination system, and the motorized translation stage inside when positioned in a rest position; 
 k) a moveable door on the protective enclosure with a pneumatic or electric cylinder to assist the opening and closing of the door; 
 l) a rear door with a liquid tight seal located opposite the moveable door that provides access to the camera image sensors, the lens, both the LED backlight and the front lighting illumination system, and the motorized translation stage when in the rest position; 
 m) an optical encoder for providing position information for ensuring that movement of the camera and the LED backlight and front lighting illumination systems reach a predetermined position where one or more images of the machine cutting tool are acquired; 
 n) whereby sensors are positioned relative to the tool, whereby a focal point of detection coincides with the center of a cross-sectional diameter so to view a feature to be measured, whereby the tool is held in a consistent location for each inspection, and whereby said LED backlight illumination system and said front lighting illumination system provide a contrasting geometric size and shape of all of the features on the tool. 
 
   
   
     2. An apparatus as recited in  claim 1 , wherein the executed control software provides for acquisition of a test image of new or used machine cutting tools and storage of the test image in memory and determining the exact position of a used machine cutting tool in the test image by extracting one or more edges based on grayscale image or grayscale sub-pixel information extracted from the grayscale image; alignment of a stored reference image of an unused machine cutting tool and the test image of the unused machine cutting tool by translation of the test image to coincide with edge points of a reference image; determining if a difference exists between the stored reference image of the machine cutting tool and the test image of the used machine cutting tool by means of a grayscale image subtraction and storing the result in a difference image; determining if the unused machine cutting tool being tested is out of position as determined by a shift in edge point information at the cutting tool tip; and generation of an error message to an external device indicating the isolation of a defective setup condition on the unused machine cutting tool tested. 
   
   
     3. An apparatus as recited in  claim 1 , wherein the executed control software provides for acquisition of a test image of a used machine cutting tool and storage of the test image in memory; determining the exact position of a used machine cutting tool in the test image by extracting one or more edges based on grayscale sub-pixel information; determining if a difference exists between the stored reference image of the unused machine cutting tool and the test image of the used machine cutting tool by means of a grayscale image subtraction and storing the result in a difference image in memory. 
   
   
     4. An apparatus as recited in  claim 1 , wherein the executed control software provides for acquisition of a test image of a used machine cutting tool and storage of the test image in memory; determining the exact position of the used machine cutting tool in the test image by extracting one or more edges based on grayscale sub-pixel information;
 determining if a difference exists between the stored reference image of a machine cutting tool and 
 the test image of the used machine cutting tool by means of a grayscale image subtraction and storing the result in a difference image; determining if the used machine cutting tool is worn by a decrease in overall length as determined by a shift in edge point information at the cutting tip, and generation of an error message to a external device indicating the isolation of a defective condition on the used machine cutting tool. 
 
   
   
     5. An apparatus as recited in  claim 1 , wherein the executed control software provides for acquisition of a test image of a used machine cutting tool and storage of the test image in memory; determining the exact position of a used machine cutting tool in the test image by extracting one or more edges based on grayscale image or grayscale sub-pixel information extracted from the grayscale image; alignment of a stored reference image of an unused machine cutting tool and the test image of the used machine cutting tool by translation of the test image to coincide with the edge points of the reference image; determining if a difference exists between the stored reference image of the machine cutting tool and the test image of the used machine cutting tool by means of a grayscale image subtraction and storing the result in a difference image; determining if the used machine cutting tool is worn by a change in the tool shape as determined by residual grayscale information in the difference image larger than a maximum number of pixels, and generation of an error message to a external device indicating a defective condition has been isolated on the used machine cutting tool. 
   
   
     6. A method for measurement and determination of key characteristics of machine cutting tools used in a CNC machining center, the method utilizing a machine vision measuring system and a computer comprising memory for executing control software embodied on a computer readable medium, to provide the steps of:
 a) storing a reference image of a new unused machine cutting tool in a memory location referenced by a specific identification code that is unique to a machine cutting tool model or type; 
 b) determining a position of the machine cutting tool in the reference image by extracting one or more edges or edge points based on a grayscale image or sub-pixel information in the grayscale image and storing the corresponding pixel locations; 
 c) determining a portion of the reference image that contains reference foreground grayscale image information that represents the machine cutting tool; 
 d) determining the portion of the reference image that contains reference background information that excludes the machine cutting tool; 
 e) acquisition of a test image of a used machine cutting tool and storage of the test image in memory; 
 f) determining the position of the machine cutting tool in the test image by extracting one or more edges or edge points based on the grayscale image or sub-pixel information in the grayscale image and storing the corresponding pixel locations in memory; 
 g) determining the portion of the test image that contains the test foreground grayscale image information that represents the machine cutting tool; 
 h) determining the portion of the test image that contains the test background grayscale image information that excludes the machine cutting tool; 
 i) creation of a background difference image by grayscale subtraction of the test background image from the reference background image; 
 j) creation of a foreground difference image by grayscale subtraction of the test foreground image from the reference foreground image; 
 k) creating and storing in memory a background list of values comprising background grayscale objects representing resulting information in the background difference image larger than a pre-determined criteria; 
 l) creating and storing a foreground list of values comprising foreground grayscale objects representing resulting information in the foreground difference image larger than a pre-determined criteria; 
 m) reduction of the stored background list by applying pre-determined size and shape filtering algorithms; 
 n) reduction of the stored foreground list by applying pre-determined size and shape filtering algorithms; 
 o) and generating an error message prompting a user that the cutting tool is broken or worn and requesting replacement of the machine cutting tool when more than a pre-determined number of values remain on the stored background and foreground lists after filtering. 
 
   
   
     7. A method as recited in  claim 6 , further comprising the steps of: determining the size of a grayscale object in the foreground difference image in pixels; determining the size of the grayscale object is larger than a pre-determined criteria; and generating an error message that prompts the user that the cutting tool is broken and requests replacement of the cutting tool. 
   
   
     8. A method as recited in  claim 6 , further comprising the steps of: determining the size of a foreground grayscale object in the foreground difference image in pixels;
 determining the size of the foreground grayscale object is smaller than a first pre-determined criteria and larger than a second pre-determined criteria; generating an error message that prompts a user that the cutting tool is worn and requesting replacement of the cutting tool in a pre-determined number of available machine cycles; receiving user input and in response, counting down the number of remaining available machine cycles, decrementing once each cycle; and when the available machine cycles reach zero generating error message to a user prompting the replacement of the cutting tool. 
 
   
   
     9. A method as recited in  claim 6 , further comprising the steps of determining the size of a foreground grayscale object in the foreground difference image in pixels; determining the size of the foreground grayscale object is smaller than a first pre-determined criteria; performing an alignment of the foreground test image to the foreground reference image; performing a second grayscale subtraction of an aligned foreground test image from the foreground reference image to create an aligned foreground difference image; determining the size of the foreground grayscale object in the aligned foreground difference image in pixels; determining the size of the foreground grayscale object is smaller than a second pre-determined criteria; generating an error message that prompts a user that the cutting tool is worn and requesting replacement of the cutting tool in a pre-determined number of available machine cycles; receive user input and in response, counting down the number of remaining available machine cycles, decrementing once each cycle; and when the available machine cycles reaches zero generating an error message to a user prompting the replacement of the cutting tool. 
   
   
     10. A method as recited in  claim 6 , further comprising the steps of: determining the size of a background grayscale object in the background difference image in pixels that is outside a region of the machine cutting tool; determining the size of the background grayscale object is larger than a pre-determined criteria; and generating an error message that prompts the user that a backlight is not functional and should be checked for operation. 
   
   
     11. A method as recited in  claim 6 , further comprising the steps of: determining the size of a background grayscale object in the background difference image in pixels that is outside region of the machine cutting tool; determining the size of the background grayscale object is smaller than a first pre-determined criteria and larger than a second pre-determined criteria; and generating an error message that prompts the user that a backlight is dirty and requires cleaning.